"For the first time," the Morningstar report says "it's the growth and aggressive funds which are the best performers over a three-year period."

The result squares with standard asset allocation lore that states riskier investments such as equities should exhibit higher returns over the long term than your safer options like cash or bonds.

In this case, however, we have to discount the four-year return figures that upend the theory with conservative beating aggressive - 5.3 per cent compared to 2.1 per cent.

Blame the GFC if you want, everyone else does, including Morningstar: "As the poor performance from the global financial crisis drops off, we are starting to see some divergence in these [asset class return] numbers."

If you accept the GFC was a one-off readjustment (as opposed to a significant and as-yet unresolved financial dislocation) then it looks like the asset-class theory is back in business.

(Please ignore the one-year figures, which again show the conservatives thumping the aggressives - too short-term by half.)

While asset allocation is usually attributed as the major influence on returns, the Morningstar figures show manager selection has also been important for KiwiSavers.

Take the three-year aggressive returns, for example, that range from 6.7 per cent (Brook Asset Management) to 16.2 per cent (Fisher Funds).

Clearly, these are differently-aggressive managers, which the one-year returns also illustrate. In the year to March 31, the three stand-out aggressive losers were Fisher Funds (-4.8 per cent), Fidelity (-6.5 per cent) and Grosvenor (-6.6 per cent for both its aggressive options) - all, incidentally, New Zealand-owned and operated.

The remaining aggressive KiwiSaver funds returned from - 2.2 per cent to a positive 3 per cent for the annual period.

As per the standard disclaimer, past returns are not indicative of future performance but they do reinforce the need for understanding the kinds of risks your KiwiSaver provider of choice may take.

Elsewhere, the Morningstar report highlights a singular change in its KiwiSaver market share ranking with Westpac moving up to number three at the expense of default provider AMP over the March quarter. While AMP hung on to its 11.4 per cent share over the three months, Westpac jumped from 11.4 per cent to 11.7 per cent, proving once again that bank trumps default anytime.

David is a freelance journalist who has covered the financial services business on both sides of the Tasman for over 15 years. He is the editor of industry website Investment News. David has edited magazines and websites for the financial advice, investment and superannuation industries.